1. Technical Field
The present invention relates in general to an improved method and system for management of multiple storage devices coupled to a common bus and in particular to an improved method and system for interconnecting multiple small computer systems interface (SCSI) host buses. Still more particularly, the present invention relates to an improved method and system for automatically altering the SCSI IDs of each target device on a crosslinked bus in response to the cross linking such that no overlapping SCSI IDs result.
2. Description of the Related Art
The small computer systems interface (SCSI) is an American National Standards Institute (ANSI) standard communications bus that includes the electrical and logical protocol specification. The first version of SCSI, SCSI-1, defines a universal parallel, system-level interface for connecting up to eight devices along a single cable, called the SCSI bus. Parallel devices (such as the majority of printers) send a group of bits at one time, as opposed to serial devices which send data one bit at a time. As a system-level interface, SCSI is very different from a device-level interface such as the Enhanced Small Device Interface (ESDI) in that SCSI is an independent and intelligent local/I/O bus through which a variety of different devices in one or more controllers can communicate and exchange information independent of what the rest of the system is doing.
SCSI has become an industry standard and is typically utilized in the computer workstation environment. SCSI may also be utilized in the personal computer environment. SCSI has become the standard for peripherals, such as disk drives, tape drives, optical storage devices, printers, scanners, etc. Like all standards, SCSI is used in a large number of installed pieces of computer equipment. Users have invested considerable money in computers and peripherals which employ the SCSI bus.
With the constant increase in the performance/price ration of computer systems, the amount of data and the number of peripherals which may need to be accommodated by the SCSI bus have increased. The problem with this increase is that there are a limited number of devices which may be coupled to a SCSI bus. For example, SCSI-1 supports up to eight devices which are generally limited to a peak throughput of five megabytes per second. So-called Fast SCSI was introduced which permitted the synchronous transfer rate to double from rates possible with SCSI-1. Thereafter, so-called wide SCSI provided a bus which was increased in width from eight bits to either sixteen or thirty-two bits in order to move more data at a faster rate and accommodate a larger number of target devices. Finally, the Fast and Wide SCSI was created utilizing both the Wide bus and the faster transfer rate to provide even greater improvement in the performance of this bus.
According to the SCSI standard for SCSI-1 and SCSI-2 hardware, SCSI signals are required to be good for a total bus length of approximately six meters when travelling through SCSI compliant cables. Fast SCSI cuts the maximum cable length in half. As those skilled in the art will appreciate, as cables get longer, the signals weaken and are more susceptible to noise and if the length of a SCSI bus must be extended, it will be necessary to utilize a so-called repeater. A repeater picks up the signal from the host adapter and reproduces that signal on the next section of cable, thereby producing an enhanced signal for devices further down the bus.
Examples of bus repeaters may be found in U.S. Pat. No. 4,604,689, U.S. Pat. No. 5,615,345 and U.S. Pat. No. 5,495,584.
Despite the utilization of repeater devices the connecting of more than one SCSI host device to a peripheral device presents several problems which are nontrivial in nature. Primarily, daisy chaining multiple host devices together limits the length of cable for each host device in a maximum length for the particular bus technology divided by the number of hosts. Secondarily, each so-called “daisy chain” type connection may cause mismatches in electrical characteristics of the cabling and increase electrical noise within the system. Finally, the interconnection of multiple host buses may create a problem in that each target device on the host bus typically includes a SCSI bus ID number and care must be taken to ensure that each section of the bus does not include SCSI ID numbers which are duplicative of numbers in another section of the bus.
It should therefore be apparent that a need exists for a method and system for interconnecting multiple SCSI host buses in a manner which is efficient and manageable.